Steerable antennas are used in a variety of applications where transmissions are to be directed at different geographical locations or targets, or conversely where it is desirable to receive signals only from a particular direction. Perhaps the two most common types of steerable antennas are reflector antennas and phased array antennas. Reflector antennas include a reflector and a feed device, such as a horn, positioned at the focal length of the reflector. The reflector is mounted on a mechanical steering device, such as a gimbal, which directs the reflector at the intended target.
Reflector antenna systems have certain advantages. For example, they are relatively inexpensive, and they can achieve a fairly large scan angle. However, such antennas also have their drawbacks. More particularly, the mechanical steering components may be relatively heavy and/or bulky for a large reflector, they take a relatively long amount of time to change directions, and they may be prone to failure. Plus, to provide a large scan angle, the antenna system requires a large amount of clearance to move the reflector.
Phased array antennas include an array of antenna elements that can be electrically phased to steer and/or shape the antenna beam. Since phased array antennas do not require a reflector or mechanical steering equipment, they typically do not suffer from the weight or clearance constraints of reflector antennas. Moreover, they provide very rapid beam steering. Yet, phased array antennas are typically more costly to implement than reflector antennas, and they tend to suffer greater signal loss as the scan angle increases. While gain elements (i.e., amplifiers) and increased numbers of antenna elements can be used to offset such signal loss and achieve desired scan angles, this increases the footprint of the array, as well as its power consumption.
Some attempts have been made in the prior art to combine the benefits of both reflector antenna systems and phased array antenna systems. More particularly, antenna element arrays have been used as the feed device for a reflector. This allows beam steering to be performed by electrically displacing the phase center of the feed array, rather than moving the reflector itself.
The basic principles involved in steering the beam of a reflector antenna are well known. However, these principles will be generally discussed herein with reference to a typical prime-focus reflector antenna system. A single feed structure is placed at the focus of the reflector and is designed such that the feed beamwidth fully illuminates the reflector. If the feed beamwidth is too wide, excess feed energy will spill over the edges of the reflector, reducing efficiency. If the feed beamwidth is too narrow, then the reflector is said to be under-illuminated and will have the gain and beamwidth commensurate with the area illuminated by the feed. In other words, under-illuminating a reflector antenna effectively creates a smaller reflector antenna which in turn has less gain and a larger beamwidth.
In actual practice, it can be desirable to slightly under-illuminate a reflector (e.g., designing the feed such that the edge of the reflector is illuminated 10 dB less than the center of the reflector) as a method to slightly reduce sidelobes and balance the efficiency of the resultant system. This is done because it is very difficult to design a reflector feed that only illuminates the reflector antenna. That is, there will almost always be some amount of spillover and amplitude taper across the reflector due to the antenna pattern of the feed. Regardless, the reflector feed is designed to produce a given beamwidth that illuminates the reflector surface in a desired manner.
If using a feed horn, for instance, this beamwidth control is achieved by proper choice of horn length and aperture. If an antenna array were used, however, the beamwidth is a function of the area of active portion of the array. Feeding more elements, or more precisely exciting a larger area of elements, will cause the beamwidth of the feed to narrow and become more directive. Either a single feed horn or a small array can be designed to properly illuminate a reflector antenna. To steer a beam in a reflector, one can displace the phase center of the feed antenna laterally, as opposed to axially, from the focus of the reflector nominally along what is referred to as the Petzval surface. The amount of beam steer is roughly equal to the angle formed by the displacement of the feed center to the center of the reflector.
To counter the disadvantages of mechanically moving a small feed antenna, attempts have been made to replace the mechanically-moved feed with a large array antenna. However, such implementations have been limited in their effectiveness. That is, if the element array is placed in the path of the antenna beam, the array has to be relatively small (typically less than 10%–15% the diameter of the reflector it is feeding as a rule-of-thumb) or severe signal blockage will occur causing undesirable degradation of the resultant antenna pattern and gain. That is, a large array will block transmitted signals coming off of the reflector, or block signals from reaching the reflector.
Yet, a small array may not be sufficient to provide desired scan angles. The array needs to be sized such that a smaller subarray, sized to provide the required beamwidth to illuminate the reflector, can be electrically “moved” by turning array elements on and off, effectively providing the same function of mechanically moving the small array. In other words, in a large array a small portion of the array can be turned on (with all other elements off) to form the required feed array size. This small subarray can be moved, or migrated, among the larger array by turning off some antenna elements in the direction the subarray is to “move” away from, and turning on others in the direction the subarray is to “move”.
This electrical movement of the feed subarray can take place much faster than in a mechanical system. Additionally, multiple clusters or subarrys of elements can be used to produce multiple beams off the reflector antenna. A disadvantage of such a system is that the required array size for large amounts of scan can be large and cause significant blockage. Since typically the active region is much smaller than the entire array, the amount of blockage and subsequent performance loss is not acceptable in many applications and may indeed be so bad as to cause the system to not function at all.
Another approach is to displace an array antenna so that it is not in front of the reflector, but is instead off to one side thereof. An example of such an antenna is disclosed in U.S. Pat. No. 6,456,252. This patent discloses a multi-feed reflector antenna system in which feed elements of a feed array are located at the focal plane of the reflector, and to the side thereof. A repeater device located at a defocused plane between the feed array and the reflector intercepts a cone angle between the feed array and the outside rim of the reflector. The repeater device includes a receiver array facing the feed array, and a transmit array facing the reflector. The repeater device receives an incoming wavefront from the feed array at the receiver array, and repeats the wavefront from the transmit array.
In the above-described system, the repeater device and feed array are both positioned to the side of the reflector. With such a side-feed arrangement, neither the repeater device nor the feed array are in the path of the antenna beam defined by the reflector. That is, they are not positioned between the reflector and the target, and thus will not block transmission signals coming off of the reflector, or signals directed at the reflector that are to be received. Yet, one drawback of using such an arrangement is that a significant amount of scan angle may be given up by offsetting the feed array from the path of the antenna beam.